The characterization of Metal complexes that establish interactions with DNA constitutes a growing research area. In addition to its potential use as therapeutic agents, they are being considered as tools for biochemistry and molecular biology. Previously a series of Ruthenium complexes of the type [Ru(II)Cl2(DMSO)2L] (where L are 5-nitrofurylsemicarbazone derivatives) have been shown to bind DNA (Cabrera et al Eur J Med Chem 2004). In order to characterize their mode of action an electrophoretic approach was carried out. Agarose gel electrophoresis using the covalently closed circular DNA form enable the visualization of modifications that could be introduced by the metal complex such as the appearance of single strand nicks, that unravels the supercoiled rendering the relaxed circular form, and/or the existence of a second scission event yielding a linear form. In addition, the appearance of conformational changes due to the formation of anomalous structures, inducing the withdrawal of negative supercoils, promotes a reduced shift migration that can be readily visualized in native gel electrophoresis. We first determined the effect of the four Ruthenium complexes on supercoiled DNA. The different members of the series exhibit variable activity. Detailed analysis of relative mobility of the DNA allows us to conclude that the Ruthenium complexes 1 to 3, introduce clear conformational changes in a dose dependent manner. This behavior is typical of compounds that alter DNA topology such as intercalators. In addition, the three above-mentioned Ruthenium complexes show the dose dependent ability to introduce a scission event yielding circular relaxed DNA form. Quantitative analysis was done by densitometry. The Ruthenium complex 4 has shown neither activity to nick nor to introduce conformational changes at the assayed doses, while complex 3 shows the clearest effect. This complex was chosen to perform a kinetic analysis, which demonstrated a rapid interaction. Even more, a nick effect could be seen even for the sample where DNA was put in contact with the Ruthenium complex 3 and immediately loaded in the agarose gel. Besides, in our conditions, the plateau was almost reached at 3 hours of incubation. Finally, the absence of oxygen did not affect the DNA cleavage by complex 3 but DMSO seems to inhibit the reaction.